Recently, a new role for metal ions in biology has been identified. Several classes of nucleic acid binding proteins have been found to contain recurring motifs of cysteine and histidine residues. It has been proposed that these sequences form domains structurally organized by metal ions coordinated to the cysteinate and histidine residues. In order to test this hypothesis and to begin structural characterization of these proteins, two approaches will be taken. First, the metal binding properties of Transcription Factor IIIA from Xenopus and the gag geneencoded nucleic acid binding proteins from the retroviruses Rauscher murine leukemia virus and Visna lentivirus will be examined. Attempts will be made to incorporate Zn2+ as well as other metal ions (Co2+, Mn2+, and Cd2+) useful for spectroscopic studies. These experiments should provide metalbinding stochiometries and data concerning the selectivity of ion binding. Spectroscopic studies will yield information about the coordination environment around the metal ions. In addition, the effects of metal ion content on the nucleic acid binding properties of the retroviral protein will be investigated. Second, peptides corresponding to individual domains will be prepared and their metal binding properties studied. Spectroscopic studies will be performed which will allow direct comparisons of the metal ion environments in the single-domain peptides with those in the intact proteins. Further, the threedimensional structures of suitable peptidemetal complexes will be determined using x-ray crystallographic and/or 1H nuclear magnetic resonance spectroscopic methods. These structures will be necessary for the development of understanding of the mechanisms by which these proteins recognize and bind nucleic acids.